In modern technology, motors are among the most commonly used elements in various applications. Many types of different motors have been developed and used, according to particular types of applications. For example, motors may be grouped into synchronous and asynchronous motors or DC or AC motors.
In any case, the need is increasingly felt to improve the efficiency of such motors. In view of the above, there are many applications in which motors might deliver a variable torque at different working time. For this reason, in order to improve efficiency, it might be useful to change the motor speed to avoid energy wastes when the motor might work in a slow running state.
Thus, electric drives with inverters are used, which are adapted to adjust the motor speed.
One of the main motor control techniques implemented by said drives is the vectorial technique, one example of which is the field orientation technique.
In short, since the DC motor has been always used as a model, due to the sharp distinction between the bias flux excitation current and the torque generating armature current, a general motor is controlled by such vectorial technique, which allows the power current to be divided into two components, known as direct current and quadrature current, which may be assimilated to the flux current and the armature current of a DC current, so that the motor may be ideally used like the above mentioned model motor. This will maximize the performances of any motor in terms of torque at various rotation speeds, speed accuracy and efficiency.
In vectorial control, direct current is similar to flux current, whereas quadrature current corresponds to armature current. Torque generation is controlled by adjusting the quadrature current, once the motor specific direct current has been determined.
A particular example is the synchronous reluctance motor where, as mentioned above, direct current control may be replaced by direct flux control.
Considering the example of the field orientation vector technique, in the control of a prior art motor, ready response can be only obtained by having direct current set to the nominal value, while quadrature current is set by a speed or torque regulator. Nevertheless, this affects efficiency, because the motor is always powered at the maximum capacity, even in case of minimal torque requirement, i.e. when the motor can run at low speed. In other words, while this arrangement keeps motor efficiency unaltered at high running speeds, efficiency is decreased, possibly to a considerable extent, at low running speeds.
For these reasons, controllers are known, which also change direct current, by adjusting the voltage at the ends of the motor as a function of load.
While this improves motor efficiency by decreasing the current supplied thereto at low running speeds, it still involves the drawback that such control is an indirect, non optimal manner to act upon direct current.
Furthermore, it is obtained by supplying predetermined direct currents in the presence of predetermined loads. Such solution is apparently not optimal, especially in the presence of mixed loads.
In other words, even when prior art motors are controlled by a field orientation vectorial technique to assimilate them to a DC motor, they still have efficiency losses at low running speeds, although control involves a change of both quadrature and direct currents.
A particular example is that of motors having controllers operating in vectorial mode, with no position or rotation speed sensor, also known as sensorless vectorial control. In this type of motors, this problem is even more serious. While the rotation speed and position of the rotor may be detected using the back electromotive force at high running speeds, the inherent impedance of the motor prevents the use of this method at low running speeds. Voltage losses cause a non negligible error, when compared with the absolute value of the back electromotive force. For this reason, at low running speeds, an additional zero-mean time-dependent voltage is added to the supply voltage. Suitable control algorithms, representing the motor-inverter assembly, such as a resolver-to-digital converter, where the motor acts as the resolver and the inverter decodes the resultant to the voltage signal altered by the added noise, provide values approximately proportional to the difference between the actual position and the estimated position, the added signal being generated at the same time as the fundamental component of the inverter.
Nevertheless, such added voltage at low running speeds induces additional work in the motor, which results in an efficiency loss.